This invention relates to a miniaturized, programmable X-ray treatment system having an electron beam source and an X-ray emitting probe for use in delivering substantially constant or intermittent levels of X-rays to a specified region and, more particularly, to the electron beam generating components of the X-ray treatment system.
In the field of medicine, radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large fixed position machines as well as small, transportable radiation generating probes. The current state of the art X-ray treatment systems utilize computers to generate complex treatment plans for treating complex geometric volumes.
Typically, these systems apply doses of radiation in order to inhibit the growth of new tissue because it is known that radiation affects dividing cells more than the mature cells found in non-growing tissue. Thus, the regrowth of cancerous tissue in the site of an excised tumor can be treated with radiation to prevent the recurrence of cancer. Alternatively, radiation can be applied to other areas of the body to inhibit tissue growth, for example the growth of new blood vessels inside the eye that can cause macular degeneration.
One type of X-ray treatment system used for such applications is disclosed in U.S. Pat. No. 5,153,900 ("'900 patent") issued to Nomikos et al., owned by the assignee of the present application, which is hereby incorporated by reference. The system disclosed in the '900 patent uses a point source of radiation proximate to or within the volume to be radiated. This type of treatment is referred to as brachytherapy. One advantage of brachytherapy is that the radiation is applied primarily to treat a predefined tissue volume, without significantly affecting the tissue in adjacent volumes.
A brachytherapy X-ray treatment system includes an X-ray source 10 shown in FIG. 1, generally comprised of an electron beam ("e-beam") source 12 and a miniaturized insertable probe assembly 14 capable of producing low power radiation in predefined dose geometries or profiles disposed about a predetermined location. The probe assembly 14 includes a shoulder 16 which provides a rigid surface by which the X-ray source 10 may be secured to another element, such as a stereotactic frame used in the treatment of brain tumors. The probe assembly 14 also includes an X-ray emitting tube 18, or "probe", rigidly secured to shoulder 16. A typical probe of this type is about 10-16 cm in length and has an inner diameter of about 2 mm and an outer diameter of about 3 mm.
Typical brachytherapy radiation treatment involves positioning the insertable probe 18 into the tumor or the site where the tumor or a portion of the tumor was removed to treat the tissue adjacent to the site with a "local boost" of radiation. In order to facilitate controlled treatment of the site, it is desirable to support the tissue portions to be treated at a predefined distance from the radiation source. Alternatively, where the treatment involves the treatment of surface tissue or the surface of an organ, it is desirable to control the shape of the surface as well as the shape of the radiation field applied to the surface.
A typical e-beam source 12 of the prior art includes a single 50 kV drift tube acceleration stage or accelerator 20, as shown in FIG. 2. The accelerator 20 includes a cylindrical body 22 comprised of a ceramic portion 24 and a metal portion 26. The accelerator 20 houses an electron gun assembly, including pins 30 which generate electrons and gun 32 which direct the electrons along a central axis 42 of the system through metal tube 38, tube opening 36, probe interface 40, and probe 18 (not shown). The metal portion 26 includes an electrically conductive ring shaped end 28, which includes Node X. The voltage at Node X is ideally 0V during operation and the voltage at Node Y is ideally -50 kV (near the electron gun), thereby providing an acceleration field for the e-beam.
In operation, the e-beam is directed to a target at the distal end of probe 18. The e-beam thus establishes a current along the axis 42. A return current path along the metallic probe 18 couples the distal end of the probe back to Node X at ring 28. The current present at Node X is ideally the beam current (I.sub.B), which is measured and an indication of the measurement is communicated to a radiation controller (not shown). The radiation controller adjusts the power supplied to the electron gun 32 as a means of adjusting the beam current and, ultimately, achieving the desired output radiation level at the end of probe 18. As a means of testing and stressing the system, a voltage of 75 kV is applied across Nodes X and Y; this is called "over-voltaging" the system.
A consequence of the single stage 50 kV accelerator 20 of a typical X-ray source 10, is that during operation stray electrons leak out from pins 30 and "avalanche" along ceramic wall 24 and metal wall 26, as shown by arrow 44. These stray electrons are typically emitted form the "triple point" 34, i.e., the point where the negative electrode 30, the vacuum within the accelerator, and the insulator 24 meet. Eventually, the stray electrons strike the end of the accelerator 20 at end 28. At very high voltages, e.g., 75 kV test voltage and 50 kV operational voltage, the electrons incident on metal end 28 have sufficient energy to cause X-rays to be emitted, resulting in unshielded X-rays which may be hazardous to those present. Additionally, the risk of high voltage hazards, such as arcing, are undesirably high while using test voltages the magnitude of 75 kV applied across a single stage. A performance related problem also results from the incident stray electrons on metal 28. The stray electrons cause a "leakage current" to be present at Node X along with the beam current. This stray current is combined with the beam current, leading to an erroneous beam current measurement and resulting calibration of the output radiation by the radiation controller. Consequently, the radiation controller erroneously adjusts the power delivered to the electron gun 32, which alters the beam current and changes the characteristics of the radiation from probe 18. The ability for the system to operate safely and perform adequately at relatively high voltages is a reflection of its "high voltage standoff" capability.
It is an object of the present invention to provide an X-ray source with improved high voltage standoff capability.
It is another object of the present invention to provide electron beam accelerator with improved high voltage standoff capability.
It is a further object of the present invention to provide a modular electron beam accelerator which requires lower voltage stress across the accelerator stages, for testing purposes and to reduce the overall size of an equivalent system.
It is a further object of the present invention to provide an X-ray source with improved radiation accuracy, by achieving improved X-ray source calibration based on accurate beam current measurements.